feat(prompts): formalize DEMO as a fourth conformance verdict

The Phase 5 reference example produced verdict DEMO in
.guides/operation/conformance.example.user-login.md because the
repository is spec-first and has no backing implementation. The
prompt's declared verdict enum was PASS/PASS-WITH-GAPS/FAIL — the
example shipped with an undocumented widening of the contract.

The Phase 5 worklog entry called this out as a known refinement
deferred to a future revision of the conformance prompt. This
commit lands that refinement.

- prompt.spec.conformance-check.yaml output rules: verdict enum
  extended to PASS | PASS-WITH-GAPS | FAIL | DEMO. DEMO is reserved
  for methodology demonstrations without a backing implementation,
  and a DEMO report MUST state which artifacts a consuming project
  would attach to produce a real PASS/PASS-WITH-GAPS/FAIL verdict.
- handle-verdict step gains a fourth branch: on DEMO, no status
  transition occurs; the demonstration boundary is recorded in the
  worklog, and the spec is linked from a README/methodology doc so
  future readers understand the verdict's methodological nature.

Now the example.user-login conformance report no longer extends an
undocumented contract — DEMO is part of the prompt's surface.

Verified: prompt still validates against prompt.schema.v2.json.

Part of Phase 6 (v0.5.1 polish) — surfaced by the post-v0.5.0 audit.